Distillation column pressure control

ABSTRACT

Methods and systems for controlling the pressure of distillation columns, for example those operating under vacuum pressure and conventionally equipped with a steam ejector system, are described. Representative distillation columns are used in the separation of thermally unstable components, such as the physical solvent sulfolane, having relatively low volatility. Such columns are employed in aromatic hydrocarbon extraction processes for the recovery of purified C 6 -C 8  aromatic hydrocarbons from a hydrocarbon feed stream (e.g., obtained from the catalytic reforming of naphtha).

CROSS REFERENCE TO RELATED APPLICATION

This application is a Division of copending application Ser. No.12/750,207 which was filed on Mar. 30, 2010, the contents of which areincorporated herein by reference thereto.

FIELD OF THE INVENTION

The present invention relates to pressure control in distillation columnoperations, and particularly those using vacuum pressures. Arepresentative example is in the separation of a physical solvent thatis subject to thermal degradation, such as sulfolane, from C₆-C₈aromatic hydrocarbons.

DESCRIPTION OF RELATED ART

Distillation processes separate components of a mixture based ondifferences in their relative volatility. Such processes are widelyemployed in industry, and especially in petroleum refining andpetrochemical production. Distillation generally involves contacting arising vapor with a falling liquid, normally in a vertically elongatedcolumn to achieve multiple theoretical equilibrium stages ofvapor-liquid contacting. Contacting and mass transfer efficiency areimproved, and the length of column needed to achieve a theoreticalequilibrium contacting stage is decreased, by incorporating contactingdevices such as trays or packing materials in the column, numerousvarieties of which are well known in the art.

As long as the formation of an azeotropic mixture is avoided within thecolumn, the vapor becomes progressively enriched in the more volatilecomponent(s) in the upward direction, while the liquid becomesprogressively enriched in the less volatile component(s) in the downwarddirection. Distillation is therefore useful, for example, in separatinghydrocarbons into fractions containing individual compounds having asimilar relative volatility or boiling point. These fractions includecrude oil-derived products of petroleum refining and petrochemicalprocessing, such as naphtha, diesel fuel, LPG, and polymers. In somecases, distillation is used to separate specific compounds from a givenimpure mixture, for example containing other compounds of the samechemical or functional class, such as alcohols, ethers, alkylaromatics,monomers, solvents, inorganic compounds, etc.

Heat input is required to generate the necessary vapor fractions atsuccessive stages and drive the distillation process. However, heatingcan be detrimental to some components that, while being desirablyseparated using distillation, exhibit thermal instability by undergoingdegradation, decomposition, polymerization, etc. In some cases,therefore, it is especially important to carry out distillation at thelowest possible temperatures and consequently the lowest possible, buteconomically feasible, distillation column pressures. Vacuumdistillation (or distillation at an absolute pressure below atmosphericpressure) is often selected for the separation of one or more thermallyunstable components. Vacuum pressure in a column can be reliablymaintained and controlled using steam ejectors as described, forexample, by Hage, H., “Steam Ejector Fundamentals,” JULY 1998 CHEMICALPROCESSING, including the use of multistaged ejectors withintercondensers between stages. In the case of vacuum distillationoperations processing high-boiling or highly condensable components, asource of a non-condensable component such as nitrogen may be introducedto the steam ejector system to facilitate pressure control, asdescribed, for example, by Lines, J. R., “Lessons from the Field-EjectorSystems,” HYDROCARBON ENGINEERING (1999). The use of such a controlscheme, however, results in losses of some of the overhead product, ordistillate, through entrainment in the nitrogen effluent. Additionally,this effluent stream often requires treatment to comply with emissionand/or safety standards.

Particular vacuum distillation operations of interest include those usedin the separation of high-boiling physical solvents from a desiredproduct that is selectively dissolved in these solvents. Physicalsolvents, as opposed to chemical solvents such as amines, do not reactchemically with the selectively dissolved product but instead promoteits physical absorption based on a high equilibrium solubility at itspartial pressure in an impure mixture (i.e., a higher Henry's lawconstant), relative to impurities in the mixture. Distillation of thephysical solvent/selectively dissolved product mixture therefore allowsrecovery of both the product in a purified form and a “lean” physicalsolvent, essentially free of the product. The recovered physical solventcan then be reused in an extraction stage for selectively dissolvingadditional amounts of the desired product.

A particular process using sulfolane (i.e., tetrahydrothiophene dioxide,also known as tetramethylene sulfone) as a physical solvent to extractor selectively dissolve a C₆-C₈ aromatic hydrocarbon product fromvarious hydrocarbon feedstocks including catalytic reformate,hydrogenated pyrolysis gasoline, coke-oven light oil, etc. is the UOPSulfolane® Process as described by Meyers, R. A., “Handbook of PetroleumRefining Processes,” The McGraw-Hill Companies, Inc. (2004), pp.2.13-2.23. This reference illustrates two process flow schemes, oneutilizing separate extraction and stripping stages, and anothercombining these stages into a single extractive distillation column.Both types of Sulfolane processes include a recovery column forseparating the C₆-C₈ aromatic hydrocarbon product from the physicalsolvent, sulfolane. Typically, this recovery column is operated undervacuum pressure to minimize thermal degradation of the solvent, asdiscussed above. A multistage steam ejector system with the introductionof nitrogen as a non-condensable material, is often used to control thevacuum pressure in this solvent recovery column.

A major consideration in the design and operation of aromatic extractionprocesses such as those described above is the energy cost. Incommercial practice, the amount of energy required may be on the orderof 1390-2100 kj/kg (600-900 BTU/lb) of aromatic hydrocarbon produced.For a typical operating unit producing about 300 kMTA (6,000 BPD) ofC₆-C₈ aromatic hydrocarbons (i.e., benzene, toluene, and xylenes), theenergy costs, namely steam, electric power, and cooling water, canaccount for over 80% of total operating costs. In contrast, solventmake-up costs are generally less than 5%, while labor/maintenance costsare typically 10%-20%. Any reduction in processing costs, andparticularly energy costs, therefore, result in a significant economicadvantage.

SUMMARY OF THE INVENTION

Aspects of present invention are associated with the discovery ofmethods for controlling the pressure of distillation columns, andespecially those operating under vacuum pressure that are conventionallyequipped with a steam ejector system as discussed above. Representativedistillation columns are those used in the separation of thermallyunstable components having relatively low volatility (e.g., a boilingpoint above 150° C. (302° F.)). Particular components include physicalsolvents used in the extractive separation of hydrocarbons such asaromatics, as well as polymer intermediates (e.g., nitriles) that arepurified using distillation but polymerize at elevated temperatures. Thephysical solvent sulfolane, for example, is subject to oxidative thermaldegradation, especially at the higher temperatures (e.g., the reboilertemperature) used in the bottom section of a distillation column. InC₆-C₈ aromatic hydrocarbon extraction processes, this solvent isrecovered by vacuum distillation from the aromatic hydrocarbon product,which is withdrawn as a vapor fraction from the upper section (e.g.,above the top contacting tray) of the distillation column, generallyreferred to as the solvent recovery column. The vapor fraction is thencondensed to provide an aromatic hydrocarbon product liquid having ahigh content of C₆-C₈ aromatic hydrocarbons.

Importantly, distillation column pressure may be conveniently controlledby bypassing, with a part of the vapor fraction withdrawn from an uppersection of the column (e.g., removed from the distillation columnoverhead above a top contacting stage), the cooler that is normally usedto cool and/or condense this vapor fraction. This part of the vaporfraction serves as a “hot vapor bypass” that, prior to entering anoverhead receiver vessel, has a significantly higher temperaturerelative to another part of the vapor fraction that passes through thecooler and then to the overhead receiver in the normal manner. Theelevated vapor pressure of the hot vapor bypass allows the columnpressure to be maintained at a desired value, typically belowatmospheric pressure but still above the vapor pressure of the condensedliquid in the overhead receiver (the overhead product liquid).Advantageously, the flow of the hot vapor bypass can be controlled tocontrol a vacuum pressure within the column (e.g., using a cascadecontrol loop with a measured pressure, relative to a setpoint pressure,in turn generating a flow setpoint for the hot vapor bypass).

Accordingly, embodiments of the invention relate to methods forcontrolling pressure in a distillation column. The methods comprisecooling and condensing, with a cooler, a first part of a vapor fraction(e.g., the net column overhead vapor) that is removed from thedistillation column (e.g., in an upper section of the column) Thecooler, and often a combination of coolers (heat exchangers), istherefore used to condense a portion of the first part of the vaporfraction removed from the column. The methods further comprisecontrolling a flow of a second part of the vapor fraction. The secondpart bypasses the cooler, or bypasses one or more of a combination ofthe coolers (heat exchangers), to control the pressure. Normally, boththe first part of the vapor fraction, after cooling and condensing it,and the second part of the vapor fraction (or hot vapor bypass) areintroduced into an overhead receiver vessel.

Particular embodiments of the invention relate to methods for purifyingaromatic hydrocarbons (e.g., aromatic extraction processes) in ahydrocarbon feed stream. The methods comprise selectively dissolving thearomatic hydrocarbons in a lean solvent (e.g., by contacting the leansolvent with the hydrocarbon feed stream in a countercurrent extractionor extractive distillation zone) to provide a rich solvent comprisingdissolved hydrocarbons that are enriched in the aromatic hydrocarbons(relative to the hydrocarbon feed stream). The methods further comprisedistilling the rich solvent in a distillation column (e.g., a vacuumdistillation column), optionally after subjecting the rich solvent tostripping to remove low boiling (relative to the aromatic hydrocarbons)hydrocarbon contaminants. Distilling of the rich solvent comprisesremoving, from an upper section of the column, a vapor fraction (e.g.,the net column overhead vapor) that is enriched (relative to the richsolvent feed to the column) in the aromatic hydrocarbons. A first partof the vapor fraction is cooled and condensed with a cooler. The flow ofa second part of the vapor fraction is controlled to control a pressureof the column. The pressure being controlled may be, for example, in theoverhead receiver or otherwise at or above a top vapor-liquid contactingstage of the distillation column.

Further embodiments of the invention are directed to pressure controlsystems for distillation columns. The systems comprise removal lines(e.g., conduits such as pipes) for first and second vapor fractions,where the lines are in fluid communication both an upper section of thedistillation column and an overhead receiver. The systems furthercomprise at least one indirect heat exchanger (e.g., a fin fan heatexchanger and/or a shell and tube heat exchanger) having a process sidein fluid communication with the first vapor fraction removal line andbeing disposed between the distillation column and the overheadreceiver. A flow controller (e.g., flow control valve) regulates a flowthrough the second vapor fraction removal line, from the upper sectionof the distillation column to the overhead receiver, which bypasses theindirect heat exchanger. This flow is regulated in response to ameasurement of pressure in the distillation column.

A number of important commercial advantages are associated with pressurecontrol methods and systems according to the present invention. Inparticular, the requirement for high pressure steam in the steamejector, normally in fluid communication with a vapor outlet of theoverhead receiver of the distillation column, can be reduced or eveneliminated. This is a substantial benefit, in view of unsuccessfulattempts by refiners to reduce high utility costs by suspending the useof the steam ejectors (e.g., by blocking or isolating this equipment),particularly in aromatic hydrocarbon recovery processes. Invariably,when the ejectors are removed from service, the vacuum column pressureis essentially governed by the vapor pressure of condensed liquid in theoverhead receiver. This pressure is not controlled and subject tovariations with varying overhead product liquid compositions. Moreover,operating at this pressure, since it is often substantially lower thanthe column design pressure, can lead to an excessive volume of vaporflow through the column that ultimately results in vapor flooding andreduced column capacity.

In addition to energy (steam) costs associated with reducing oreliminating the operation of the steam ejector(s) conventionally used toinduce and control a vacuum pressure, one or more associated processeffluents may also be reduced or eliminated. In particular, therequirements for treatment of wastewater, and optionally a nitrogeneffluent, generated from the steam ejector system, can be diminished.The wastewater results from condensing of steam used to drive theejector(s) and the nitrogen effluent results from the nitrogen input tomaintain a flow of non-condensables for conventional pressure control.Both of these streams are contaminated to some extent with components(e.g., C₆-C₈ aromatic hydrocarbons) present in the overhead vapor of thevacuum distillation column. According to other embodiments, the steamejector(s) may be used only discontinuously during the operation of thedistillation column, for example during startup and/or at other discreettimes when evacuation of non-condensable vapors from the column isdesired. In any event, even with discontinuous use, utility costs andtreatment costs for generated wastewater and waste gas are realized. Theuse of pressure control methods described herein therefore providereliable control of pressure in the sub-atmospheric (vacuum) range. Themethods thereby offer a number of significant benefits associated withvacuum distillation operations and in many cases a convenientalternative to steam ejector systems.

These and other aspects and features relating to the present inventionare apparent from the following Detailed Description.

BRIEF DESCRIPTION OF THE DRAWING

The Figure depicts a representative vacuum distillation column that maybe operated according to methods and with control systems as describedherein.

The Figure is to be understood to present an illustration of theinvention and/or principles involved. Details including some of theequipment (e.g., the reboiler) and some of the instrumentation andcontrol loops, as well as other items not essential to the understandingof the invention are not shown. The broken lines 12 a and 22 b areintended to represent methods according to additional embodiments of theinvention, but it will also be readily apparent to one of skill in theart having knowledge of the present disclosure that methods and systemsfor controlling distillation column pressure according to various otherembodiments of the invention, will have configurations and componentsdetermined, in part, by the specific application.

DETAILED DESCRIPTION

As discussed above, the methods described herein relate to the controlof pressure in distillation columns and particularly those operatingunder vacuum pressure, which is often desired in the separation bydistillation of an overhead, vapor fraction having a bubble point (ortemperature at which the first bubble of vapor forms at atmosphericpressure) that is well above the temperature usually achieved by coolingof this overhead vapor fraction with industrial cooling water. Thebubble point of the vapor fraction is therefore generally above about50° C. (122° F.), typically above about 100° C. (212° F.), and oftenabove about 150° C. (302° F.)). In the case of distillation to achievean overhead vapor fraction comprising substantially all (e.g., greaterthan 99%) of a single component or compound then these temperatureranges apply to the boiling point of that compound. Representativevacuum distillation operations may also be characterized in that one ormore of the components in the mixture fed to the column is thermallyunstable, meaning that it is susceptible to significant oxidation,degradation, decomposition, polymerization, etc. at elevatedtemperatures and particularly the highest temperatures that would beobtained in the bottom section (e.g., the reboiler), if the distillationcolumn were operated at atmospheric pressure or above. In the case ofsulfolane, for example, a commercially prohibitive, oxidative thermaldecomposition rate is encountered at its normal boiling point of 285° C.(545° F.). However, a significantly lower bottom column temperature, forexample in the range from about 165° C. (329° F.) to about 205° C. (401°F.), is typically achieved using vacuum distillation.

Particular, non-limiting, vacuum distillation operations include thoseused in the separation of a hydrocarbon product, as a more volatilefraction of a feed stream to the distillation column, from a physicalsolvent, as a less volatile fraction of the feed stream. Physicalsolvents include those used to selectively extract or dissolve desiredhydrocarbon products of interest, in an upstream extraction orextractive distillation zone. For example, a hydrocarbon productcomprising C₆-C₈ aromatic hydrocarbons (benzene, toluene, and mixedxylene isomers (ortho-, meta-, and para-xylene)) may be recovered froman impure mixture of hydrocarbons in a hydrocarbon feed stream (e.g.,comprising a catalytic reformate, hydrogenated pyrolysis gasoline,and/or coke-oven light oil) by extraction, or otherwise extractivedistillation, into a physical solvent comprising sulfolane (i.e.,tetrahydrothiophene dioxide, also known as tetramethylene sulfone) asdiscussed above. Other representative physical solvents includepropylene carbonate, tributyl phosphate, and methanol. Still othersinclude alkyl- and alkanol-substituted heterocyclic hydrocarbons such asalkanolpyridines (e.g., 3-(pyridin-4-yl)-propan-1-ol) andalkylpyrrolidones (e.g., n-methyl pyrrolidone), as well as dialkylethersof polyethylene glycol (e.g., polyethylene glycol dimethyl ether).Separation of the physical solvent, for reuse in the extraction orextractive distillation of the desired hydrocarbons, occurs in a solventrecovery column that typically operates under vacuum pressure tominimize oxidative degradation and/or other types of thermaldecomposition of the physical solvent.

A representative distillation column illustrating various aspects of theinvention is shown in the Figure. The distillation column may be, forexample, a typical solvent recovery column, as discussed above,operating under vacuum pressure. A feed stream 10 comprising first andsecond fractions to be separated is passed to column 100. The firstfraction may be, for example, the more volatile vapor fraction 12 (orcombination of 12 and 12 a) removed from the upper section (e.g., abovea top contacting tray (not shown) or above internal packing material(not shown)) of the column 100 and therefore having a lower bubble pointthan the second fraction, which may be the less volatile liquid fraction14 removed from the lower section (e.g., below a bottom contacting tray(not shown) or below internal packing material (not shown)) of thecolumn 100. The first and second fractions may independently comprisesubstantially a single component or compound, in which case the bubblepoint of the fraction is substantially the boiling point of thatcomponent or compound. According to a representative embodiment directedto a solvent recovery column in an aromatic hydrocarbon extractionprocess, the first fraction may comprise a mixture of components, andparticularly a mixture of C₆-C₈ aromatic hydrocarbons, which may bepresent in a combined amount generally greater than about 95%, and oftengreater than about 99%, by weight of the fraction. The second fractionmay comprise substantially (e.g., greater than about 99% by weight) thephysical solvent (e.g., sulfolane).

The feed stream 10 passing to column 100 therefore comprises at least afirst, more volatile component or compound having a relatively lowboiling point and a second, less volatile component or compound having arelatively high boiling point. The vapor fraction 12 is thereforeenriched in the first component, relative to feed stream 10, while theliquid fraction 14 is enriched in the second component, relative to feedstream 10. In a representative embodiment directed to an aromatichydrocarbon extraction process in which the column 100 is a solventrecovery column, therefore, the first component may be sulfolane and thesecond component may be selected from C₆-C₈ aromatic hydrocarbons.According to some embodiments, vapor fraction 12 of distillation column100 may be removed as, and therefore comprise, more than one streamexiting the upper section of this column. As shown in the Figure, forexample, the vapor fraction may be a combination of 12 and 12 a, shownwith a broken line, taken from different points in the upper section ofcolumn 100. It is also understood that feed stream 10 may be composed ofa number of separate streams (not shown) passing to column 100,optionally at different locations (heights or tray numbers) along thecolumn. It is further understood that other vapor and liquid or combinedvapor/liquid fractions (not shown) may be removed from column 100,optionally at different locations.

According to embodiments of the invention, a first part 16 of vaporfraction 12 is passed through coolers 18, 20. As illustrated in theFigure, coolers 18, 20 may be, respectively, a first indirect heatexchanger 18 (e.g., a fin fan cooler) using air as a cooling medium anda second indirect heat exchanger 20 (e.g., a trim condenser) using wateras a cooling medium. First part 16 of vapor fraction 12 is thereforecooled and condensed after removal from column 100. The condensing offirst part 16 of vapor fraction 12 generally refers to at least aportion of this part being condensed to liquid that is at equilibriumwith its vapor (after passing through cooler 20) at approximatelycooling water temperature, typically from about 25° C. (77° F.) to about45° C. (113° F.). The condensed liquid, together with additional liquidcondensed in the overhead receiver 26 from second part 22 (or 12 a) ofvapor fraction 12 (or combination of 12 and 12 a), optionally afterpassing through cooler 20 (via broken line 22 b), is recovered asoverhead product liquid 28, for example an aromatic hydrocarbon productliquid having a content of C₆-C₈ aromatic hydrocarbons in excess of 99%by weight. These aromatic hydrocarbons are often separated by downstreamdistillation into fractions enriched in benzene, toluene, and xylenes(relative to the aromatic hydrocarbon product liquid) with the latteroften being further separated and isomerized to recover and purifydesired xylene isomers (e.g., para-xylene for the production of purifiedterepthalic acid (PTA)). Product liquid 28 refers to the net overheadliquid removed from column 100, and in many cases a significant amountof the total liquid flow to overhead receiver 26 is returned to column100 as reflux (not shown).

The flow of a second part 22 (or 12 a) of vapor fraction 12 (orcombination of 12 and 12 a) is controlled using a controller such as hotvapor bypass control valve 24 and this flow bypasses both of the coolers18, 20, as shown with line 22 a, or according to alternativeembodiments, only the first cooler 18, as shown with broken line 22 b.In either case, the bypassing of at least one of coolers 18, 20 resultsin second part 22 (or 12 a) of vapor fraction 12 (e.g., at hot vaporbypass control valve 24 and immediately upstream of overhead receiver26), to be hotter than first part 16 of vapor fraction 12, after passingthrough coolers 18, 20. For example, while this temperature of firstpart 16 after cooling may approximate industrial cooling watertemperature as discussed above, the temperature of second part 22 mayapproximate a top temperature in column 100, having a representativerange, in the case of a solvent recovery column in an aromatichydrocarbon extraction process, from about 75° C. (167° F.) to about100° C. (212° F.). According to a representative embodiment, the coolerfirst part 16 of vapor fraction 12, after cooling and condensing, isintroduced into overhead receiver 26 at a location below that at whichsecond part 22 (or 12 a) is introduced, through 22 a, as illustrated inthe Figure. This particular routing of the cooler and hotter parts ofthe vapor fraction allows a temperature gradient to form in the liquidcondensed in overhead receiver 26, minimizing disturbances fromexcessive condensation at the top of this vessel.

Second part 22 (or 12 a) of vapor fraction 12 is therefore a hot vaporbypass, having a vapor pressure above that of condensed liquid inoverhead receiver 26. Both of these vapor pressures may be belowatmospheric pressure, or otherwise the vapor pressure of the hot vaporbypass may be above atmospheric pressure while the vapor pressure of thecondensed liquid, and consequently the vapor pressure of the first part16 of vapor fraction 12 after cooling, is below atmospheric pressure. Inany event, the vapor pressure of the hot vapor bypass is equal to orabove a setpoint pressure, for example controlled in the vapor space ofoverhead receiver 26 at point A. This pressure control point correspondsto that associated with the conventional operation of steam ejector 32,which, during its operation, is in fluid communication with a vaporoutlet 30 of overhead receiver 26. An alternative setpoint pressure iscontrolled in an upper section of column 100 at point B. Arepresentative setpoint pressure is from about 0.1 bar (1.5 psi) toabout 1 bar (14.5 psi), corresponding to the operating pressure ofcolumn 100.

As discussed above, the hot vapor bypass advantageously provideseffective pressure control without relying on continual operation ofsteam ejector 32, thereby saving significant utility and waste streamtreatment costs. In particular, the equal or higher vapor pressure ofthe hot vapor bypass, relative to that of the setpoint pressure, allowshot vapor bypass control valve 24 to control this setpoint pressure bycontrolling the flow of second part 22 (or 12 a) of vapor fraction 12 inresponse to a measured pressure (e.g., at point A), or deviation betweenthe setpoint pressure and measured pressure, in column 100 or in itsoverhead receiver 26. According to one control scheme, the measuredpressure, relative to the pressure setpoint, is used as a basis for apercentage opening or a flow setpoint for the hot vapor bypass, namelythe second part 22 (or 12 a) of vapor fraction 12, through hot vaporbypass control valve 24. In an alternative control scheme, a measureddifferential pressure (e.g., the differential pressure between points Aand B) is used as a basis for a valve opening percentage or a flowsetpoint for the hot vapor bypass. In yet another embodiment, hot vaporbypass control valve 24, having a percentage opening or a flow setpointbased on a differential pressure, operates in conjunction with mainvapor control valve 34, prior to (upstream of) first indirect heatexchanger 18 (e.g., a fin fan cooler). Main vapor flow control valve 34may have a flow setpoint based on a measured pressure, for example atpoint B. The combination of differential pressure control, using hotvapor bypass control valve 24 and pressure control, using main vaporcontrol valve 34, therefore serves to maintain a desired pressure incolumn 100, even without the use of steam ejector 32.

Particular embodiments of the invention relate to the use of the columnpressure control methods, as described herein, in an aromatichydrocarbon extraction process. Accordingly, aspects of the inventionare associated with methods for separating or purifying C₆-C₈ aromatichydrocarbons from an impure hydrocarbon feed stream (i.e., having alower content of C₆-C₈ aromatic hydrocarbons compared to a C₆-C₈aromatic hydrocarbon product liquid stream obtained from the process).The methods comprise selectively dissolving the aromatic hydrocarbons ina lean solvent to provide a rich solvent comprising dissolvedhydrocarbons that are enriched in the aromatic hydrocarbons (relative tothe hydrocarbon feed stream) due to the selectivity of the solvent forthese hydrocarbons. Contact between the lean solvent and the hydrocarbonfeed stream is normally performed in a countercurrent extraction zone ora countercurrent extractive distillation zone, with the rich solventliquid exiting the bottom of an extraction or extractive distillationcolumn. The rich solvent, optionally after being subjected to stripping,is then distilled in a solvent recovery column. According to aspects ofthe invention, the pressure of this column may be controlled undervacuum pressure according to the methods discussed above. As noted, aparticular process of interest involves the purification of C₆-C₈aromatic hydrocarbons, obtained from the catalytic reforming of naphtha(e.g., from the overhead of the reformate splitter) using sulfolane asthe selective solvent.

Overall, aspects of the invention are associated with processes andsystems for controlling the pressure of distillation columns, andparticularly those operating under vacuum pressure such as solventrecovery columns used in aromatic hydrocarbon extraction processes.Those having skill in the art, with the knowledge gained from thepresent disclosure, will recognize that various changes could be made inthe above processes and systems without departing from the scope of thepresent disclosure. Mechanisms used to explain theoretical or observedphenomena or results, shall be interpreted as illustrative only and notlimiting in any way the scope of the appended claims.

1. A method for purifying aromatic hydrocarbons in a hydrocarbon feed stream, the method comprising: (a) selectively dissolving the aromatic hydrocarbons in a lean solvent, to provide a rich solvent, and (b) distilling the rich solvent in a distillation column operating at sub-atmospheric pressure, optionally after subjecting the rich solvent to stripping, wherein step (b) comprises (i) removing a net overhead vapor enriched in the aromatic hydrocarbons, relative to the rich solvent, from an upper section of the column, (ii) cooling and condensing a first part of the net overhead vapor with a cooler, and (iii) controlling the flow of a second part of the net overhead vapor, bypassing the cooler, to control a pressure measured at or above a top vapor-liquid contacting stage in the distillation column.
 2. The method of claim 1, wherein step (a) comprises contacting the lean solvent with the hydrocarbon feed stream in a countercurrent extraction or extractive distillation zone.
 3. The method of claim 1, wherein the lean solvent and the rich solvent comprise tetrahydrothiophene dioxide.
 4. The method of claim 1, wherein the aromatic hydrocarbons are C₆-C₈ aromatic hydrocarbons.
 5. The method of claim 4, wherein at least a portion of the hydrocarbon feed stream is obtained from catalytic reforming of naphtha.
 6. The method of claim 4, further comprising introducing the first and second parts of the net overhead vapor into an overhead receiver vessel and removing an aromatic hydrocarbon product liquid from the overhead receiver vessel.
 7. The method of claim 6, further comprising separating the aromatic hydrocarbon product liquid into fractions enriched in benzene, toluene, and xylenes.
 8. A pressure control system for a distillation column operating at sub-atmospheric pressure, the system comprising: (a) first and second vapor fraction removal lines in fluid communication with an upper section of the distillation column; (b) an overhead receiver in fluid communication with both the first and second vapor fraction removal lines; (c) at least one indirect heat exchanger having a process side in fluid communication with the first vapor fraction removal line, between the distillation column and the overhead receiver; and (d) a controller for regulating a flow through the second vapor fraction removal line in response to a measured pressure in the overhead receiver or in the distillation column, wherein the second vapor fraction removal line bypasses the indirect heat exchanger. 